In petroleum exploration, various parameters of an earth formation may be measured to estimate a presence of hydrocarbon, such as oil or gas. Electrical earth borehole logging is well known and various devices and techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, called “galvanic” devices, electrodes emit current into the earth formations in order to determine resistivity. One of the simplest forms of galvanic devices is the so-called “normal” device where a current electrode emits a current through the earth formation to a remote return location, and a voltage electrode measures the potential due to that current with respect to the remote reference location. In the second category (inductive measuring tools), an antenna within the measuring instrument may induce a current flow within the earth formation. Resistivity can be determined by measuring either the magnitude or the attenuation caused by propagation of this current by same antenna or separate receiver antennas.
Also, identifying the location of shale layers and knowing the proportion of shale in the formation is important for exploring and developing a reservoir. The shale volume may be involved in wellbore stability analysis, rock classification, computation of volumetric composition of the formation, including hydrocarbon saturation and water saturation. Shale volume may be derived from formation investigation logs using measurements involving gamma ray, spontaneous potential, neutron-density combination, and resistivity.